Pattern of continuous muscle fibre activity depending on fibre stretch and stimulation frequency.
Isolated frog muscle fibres of four different lengths (L0, initial; L1, 15%; L2, 25%; and L3, 35% stretched vs. L0) were stimulated continuously (3 min) by suprathreshold rectangular pulses at three different frequencies (f1 = 5 Hz, f2 = 6.7 Hz and f3 = 10 Hz). Using a pair of electrodes mounted at a fixed distance, the action potential at two different sites was recorded extracellularly and the propagation velocity (PV) of excitation was calculated. Throughout the trials two kinds of activity (continuous and intermittent) were observed. Two types of muscle fibres, slow (SMFs) and fast (FMFs), were distinguished depending on the rate of decrease in the PV during the period of continuous fibre activity. The duration of this period decreased with the increase of L and f. The continuous activity was followed by intermittent activity. The variety of alternations of rest periods (i.e. failure of potentials and periods of activity) characterized the pattern of SMF and FMF activity evoked by the protocol used. A coefficient of activity (k), i.e. the ratio between the number of action potentials and the number of stimulus pulses delivered for a determined time interval, was used to estimate the specificity of this pattern. A decrease in k was observed with an increase of L and f, and was differently expressed for the two fibre types. The stretch of SMFs contributed more to the shortening of the period of continuous activity and the decrease in k than did the increase of stimulation frequency. For FMFs the effect of the two factors (L and f) was identical. It is suggested that the disturbances of sarcolemmal and t-system membrane excitability during repetitive stimulation and stretch are fibre-type dependent and underlie the pattern of muscle fibre activity.
Radicheva
N
N
Institute of Biophysics, Bulgarian Academy of Sciences, Sofia, Bulgaria.
Mileva
K
K
Vydevska
M
M
D000200:Action Potentials; D000818:Animals; D004558:Electric Stimulation; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology; D011895:Rana ridibunda